Transistors formed over a glass substrate or the like are typically manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field effect mobility, they can be formed over larger glass substrates. On the other hand, although a transistor manufactured using polycrystalline silicon has high field-effect mobility, it has a disadvantage of not being suitable for a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic appliance or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel of a display device.
As for an oxide semiconductor used in such a transistor, there is description as follows: an oxide semiconductor is insensitive to impurities; there is no problem when a considerable amount of metal impurities is contained in the film; and, soda-lime glass which contains a large amount of alkali metal such as sodium and is inexpensive can also be used (see Non-Patent Document 1).